Attach the signal wires for the trailer brakes to switch an electric motor that is linked to the trailer wheels. The motor would be used only as a braking-assist, powering some arrangement of trailer mounted batteries. Upon arriving home, plug the trailer into your wall socket to power the house or to sell back to the electric company.

This would be even more useful when arriving at a destination without electricity, such as a campsite, Florida after a hurricane, or Baghdad most of the time. Your mileage may vary depending on the weight of the batteries, cost of fuel, and cost of electricity. Do take care to make the trailer fit aerodynamically within in the wake of the auto, and if possible try to plan out a route that runs mostly down hill. Consider getting a steam car.